Replace mk_foo calls with infer_foo where possible.

There are several `mk_foo`/`intern_foo` pairs, where the former takes an
iterator and the latter takes a slice. (This naming convention is bad,
but that's a fix for another PR.)

This commit changes several `mk_foo` occurrences into `intern_foo`,
avoiding the need for some `.iter()`/`.into_iter()` calls. Affected
cases:
- mk_type_list
- mk_tup
- mk_substs
- mk_const_list

compiler/rustc_middle/src/mir/mod.rs

@@ -2525,14 +2525,12 @@ impl<'tcx> ConstantKind<'tcx> {
         }
 
         let hir_id = tcx.hir().local_def_id_to_hir_id(def.did);
-        let parent_substs = if let Some(parent_hir_id) = tcx.hir().opt_parent_id(hir_id) {
-            if let Some(parent_did) = parent_hir_id.as_owner() {
-                InternalSubsts::identity_for_item(tcx, parent_did.to_def_id())
-            } else {
-                tcx.mk_substs(Vec::<GenericArg<'tcx>>::new().into_iter())
-            }
+        let parent_substs = if let Some(parent_hir_id) = tcx.hir().opt_parent_id(hir_id)
+            && let Some(parent_did) = parent_hir_id.as_owner()
+        {
+            InternalSubsts::identity_for_item(tcx, parent_did.to_def_id())
         } else {
-            tcx.mk_substs(Vec::<GenericArg<'tcx>>::new().into_iter())
+            tcx.intern_substs(&[])
         };
         debug!(?parent_substs);